An image display device is roughly classified into an impulse type display device and a hold type display device. In a CRT (Cathode Ray Tube) mentioned of as an example of an impulse type display device, a screen is scanned by an electron gun and a display is produced only in pixels that electron beams have reached. In contrast to this, in a liquid crystal display device or an organic electroluminescence display device mentioned of as a hold type display device, a frame of an image signal is updated at a fixed period and when a display of an image of a certain first frame is specified, the display of the image of the first frame is held until a display of an image of a second frame that follows is specified. Compared to an impulse type display device, a hold type display device has various characteristics, such as that image distortion is unlikely to occur.
However, a liquid crystal display device has a problem that response is slow. That is, it takes time for an actual display value in a liquid crystal display device to reach a target display value after the target display value of an image of a certain frame is specified. There may be a case where the required time exceeds a period at which a frame is updated. Consequently, when a motion picture in which images changes rapidly is displayed on the screen of a liquid crystal display device, there may be a case where blur appears in the motion picture.
As a technique intended to solve such a problem, the overdrive technique is publicly known. According to the overdrive technique, when a certain pixel on the screen of a liquid crystal display device is focused on, if image data G2 corresponding to a target display value in the next second frame is different from image data (luminance) G1 corresponding to a target display value in a certain first frame, the image data G2 is corrected and then, corrected image data G2′ is given to the liquid crystal display device. At the time of the correction, when “G1<G2”, G2 is corrected so that “G2<G2′” and when “G1>G2”, then G2 is corrected so that “G2>G2′”. By providing an image signal processing device that outputs an image signal to a liquid crystal display device after processing image data of each frame of the image signal as described above, it is made possible for the actual display value to reach the target display value quickly in the liquid crystal display device.
There have been made various proposals relating to the overdrive technique. In the invention disclosed in patent document 1, a lookup table, in which each value of the above-mentioned image data (G1, G2) and the corrected image data G2′ are associated with each other and stored, is used and the corrected image data G2′ corresponding to the image data (G1, G2) is output from the lookup table for each pixel. In this case, for example, when the image data is 8 bits and the display value is in the range of 0 to 255, the number of kinds of the data (G1, G2) to be input to the lookup table is 65,536 (=256×256), and therefore, it is necessary to use a memory of large capacity as the lookup table.
Patent documents 1, 2 disclose the invention that aims at reduction in the capacity of a memory used as the lookup table. In the invention disclosed in these documents, only the high order bits of the respective data G1, G2 are input to the lookup table, and the corrected image data G2′ is acquired by interpolation calculation based on the data output from the lookup table.
Patent document 1: Japanese Unexamined Patent Publication (Kokai) No. 2005-352155
Patent document 2: Japanese Unexamined Patent Publication (Kokai) No. 2004-004829